Title

Author

Defense Date

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Anatomy & Neurobiology

First Advisor

Linda Phillips

Abstract

Traumatic brain injury (TBI) affects 1.5 million Americans annually and is a major health concern. Increasing evidence suggests that the brain extracellular environment regulates plasticity and synaptic recovery following TBI. Here we have focused on phosphacan/RPTPβ, an alternatively spliced group of chondroitin sulfate proteoglycans which are prominent within the mature brain extracellular matrix (ECM). Previous studies show that phosphacan/RPTPβ influences neuronal migration, adhesion, neurite outgrowth, and morphogenesis. However, our understanding of how these important ECM components are involved in recovery from brain trauma remains unclear. In the present study, we used unilateral entorhinal cortex lesion (UEC), a model which induces robust hippocampal reactive plasticity, to investigate the role(s) of phosphacan/RPTPβ isoforms in adaptive synaptogenesis after TBI. Using detailed protein and mRNA quantification, immunohistochemistry, and qualitative ultrastructural analyses, we show elevated phosphacan expression in the deafferented hippocampus at the early degenerative phase and during the subsequent period of active sprouting. By contrast, the receptor variant sRPTPβ is persistently elevated in hippocampus over the first two weeks following UEC. We have further characterized a process for validating appropriate reference genes for quantitative real-time RT-PCR studies of plasticity and recovery after TBI. From these studies we conclude that injury model, brain region, survival period and correlative protein expression are critical factors which must be considered for reference gene selection. Finally, we investigated functional implications of sRPTPβ increase during reactive synaptogenesis, showing that the sRPTPβ substrate β-catenin, an important cytoskeletal regulator, is altered in hippocampus during injury-induced plasticity. Together, these results support a role for phosphacan/RPTPβ in both degenerative and regenerative phases of reactive synaptogenesis. Phosphacan may promote adaptive plasticity at earlier post-injury phases through interactions with adhesion molecules or growth factors in the extracellular space. The prolonged increase in sRPTPβ after UEC, along with its localization at postsynaptic profiles, suggests that this isoform may work with intracellular substrates to influence spine morphogenesis and/or stabilization of new synapses. Gaining a better understanding of the roles of ECM components in recovery from TBI will be an essential part of defining the difference between injuries where recovery is successful, and those where recovery fails.